FIG. 4 is a circuit diagram of a typical inverter circuit. The inverter circuit 100 includes a first direct current (DC) input terminal 110 connected to a first DC power supply (not shown), a second DC input terminal 120 connected to a second DC power supply (not shown), a transformer 130, a first switch transistor 140, a second switch transistor 150, a pulse generator 160, a pulse width modulation (PWM) circuit 170, and a filter circuit 180. The transformer 130 includes a first primary winding 131, a second primary winding 132, and a secondary winding 133. The pulse generator 160 includes an input terminal 161, a first output terminal 162, and a second output terminal 163. The PWM circuit 170 includes an output terminal 171 configured to provide a square pulse. The first and second switch transistors 140, 150 are N-channel metal-oxide-semiconductor field-effect transistors (NMOSFETs).
The first primary winding 131 and the second primary winding 132 share a tap 135. The tap 135 is connected to the first DC input terminal 110.
The first switch transistor 140 includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” is connected to ground. The drain electrode “D” is connected to the other tap (not labeled) of the first primary winding 131 of the transformer 130. The gate electrode “G” is connected to the first output terminal 162 of the pulse generator 160.
The second switch transistor 150 includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” is connected to ground. The drain electrode “D” is connected to the other tap (not labeled) of the second primary winding 132 of the transformer 130. The gate electrode “G” is connected to the second output terminal 163 of the pulse generator 160.
The input terminal 161 of the pulse generator 160 is connected to the output terminal 171 of the PWM circuit 170 for receiving the square pulse. The pulse generator 160 generates two pulse driving signals with opposite phases according to the received square pulse, and provides the two pulse driving signals to the first switch transistor 140 and the second switch transistor 150, respectively.
The second DC input terminal 120 provides operation voltages to the pulse generator 160 and the PWM circuit 170 respectively. The filter circuit 180 connects between the tap 135 and ground. The filter circuit 180 includes a resistor (not labeled) and a capacitor (not labeled) connected in series.
When the pulse driving signal at the first output terminal 162 of the pulse generator 160 is a high level voltage and the pulse driving signal at the second output terminal 163 of the pulse generator 160 is a low level voltage, the first switch transistor 140 is turned on and the second switch transistor 150 is turned off. Thus a first current path is formed sequentially through the first DC input terminal 110, the tap 135, the first primary winding 131 of the transformer 130, and the first switch transistor 140. A first current is formed when the first DC power supply provided to the first DC input terminal 110 is connected to ground via the first current path. The first current flowing through the first current path linearly increases until the electromagnetic induction generated in the first primary winding 131 reaches a predetermined maximum threshold.
When the pulse driving signal at the first output terminal 162 of the pulse generator 160 is a low level voltage and the pulse driving signal at the second output terminal 163 of the pulse generator 160 is a high level voltage, the first switch transistor 140 is turned off and the second switch transistor 150 is turned on. Thus a second current path is formed sequentially through the first DC input terminal 110, the tap 135, the second primary winding 132 of the transformer 130, and the second switch transistor 150. A second current is formed when the first DC power supply provided to the first DC input terminal 110 is connected to ground via the second current path. The second current flowing through the second current path linearly increases until the electromagnetic induction generated in the second primary winding 132 reaches a predetermined maximum threshold.
When current flows through the first primary winding 131 or the second primary winding 132, electromagnetic induction at the secondary winding 133 generates an alternating current (AC) voltage between two taps (not labeled) of the secondary winding 133. The AC voltage is used for driving a load (not shown). The load may for example be lamps (not shown) of a backlight module of a liquid crystal display device. When the lamps are driven, they light up.
The circuit configuration of the inverter circuit 100 is completely symmetrical, thus the parameters of the same electronic elements of the inverter circuit 100 (e.g. the first switch transistor 140 and the second switch transistor 150) must be the same. But in fact, the parameters of the same electronic elements of the inverter circuit 100 are a little different, due to permissible variation (tolerance) in the specification for the electronic element. For example, a resistance between a source electrode and a drain electrode of a transistor is 30±5 milliohms (mΩ) according to the specification for the transistor. When a resistance between the source electrode “S” and the drain electrode “D” of the first switch transistor 140 is 35 mΩ, and a resistance between the source electrode “S” and the drain electrode “D” of the second switch transistor 150 is 25 mΩ, the current flowing through the first primary winding 131 of the transformer 130 is smaller than the current flowing through the second primary winding 132 of the transformer 130.
FIG. 5 is a waveform diagram of current flowing through the tap 135 of the transformer 130. Vg1 and Vg2 represent the pulse driving signal at the output terminal 162 of the pulse generator 160 and the pulse driving signal at the output terminal 163 of the pulse generator 160, respectively.
During a time t1, Vg1 is a low level voltage and Vg2 is a high level voltage (i.e. the first switch transistor 140 is turned off and the second switch transistor 150 is turned on), and the second current flows through the tap 135 of the transformer 130. The second current linearly increases and finally reaches a maximum current I1.
During a time t2, Vg2 turns to a low level voltage (i.e. the second switch transistor 150 is turned off), and the second current reverses instantaneously. Then the reversed second current linearly increases and but does not reach zero in time t2.
During a time t3, Vg1 turns to a high level voltage (i.e. the first switch transistor 140 is turned on), and the first current flows through the tap 135 of the transformer 130. The first current firstly counteracts the reversed second current of the transformer 130, i.e., a mixed current flowing through the tap 135 firstly reaches zero. Then the first current linearly increases and finally reaches a maximum current I2. The difference between the maximum current I1 and the maximum current I2 is 0.6 A.
During a time t4, Vg1 turns to a low level voltage (i.e. the first switch transistor 140 is turned off), and the first current flowing through the tap 135 reverses instantaneously. The reversed first current then linearly increases. Because the reversed first current is smaller than the reversed second current, the reversed first current increases to zero in time t4.
Then the inverter circuit 100 repeats the above process. We can conclude that when the resistance between the source electrode “S” and the drain electrode “D” of the first switch transistor 140 is larger than the resistance between the source electrode “S” and the drain electrode “D” of the second switch transistor 150, and the first current is smaller than the second current. A temperature of the second primary winding 132 is higher than a temperature of the first primary winding 131. Thus the transformer 130 may become damaged or even destroyed after working for a long time.
When the resistance between the source electrode “S” and the drain electrode “D” of the first switch transistor 140 is smaller than the resistance between the source electrode “S” and the drain electrode “D” of the second switch transistor 150, the above-described problem also exists.
It is desired to provide a new inverter circuit which can overcome the above-described deficiencies.